Point-controlled contention arbitration in multiple access wireless LANs

ABSTRACT

A power management method that divorces multicast frames for power save stations from DTIM beacons. An access point associates power save stations with a multicast group. Each station has an associated power save interval, the station being in a power save state during the interval, otherwise in an active state. The access point maintains a timer for each station in order to determine whether the station is in an active or power save state. The timer can be reset whenever the station sends an inbound frame. When the access point receives a multicast frame for the group, it buffers the frame until the timer for each member of the group expires, thereby ensuring each member of the group is in an active state, and broadcasts the multicast frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.09/953,820 filed on Sep. 12, 2001, which claims the benefit of U.S.Provisional Application No. 60/252,717 filed Nov. 22, 2000.

BACKGROUND OF THE INVENTION

The present invention is directed to channel access methods that provideQuality of Service (“QoS”) for shared communications, more specificallyto channel access methods that provide Quality of Service on IEEE 802.11wireless networks.

The abbreviations and acronyms used in this application are well knownto those skilled in the art and can be readily located in the IEEE802.11 standard, or in the IEEE 802.11E QoS baseline proposal. SeeMichael Fischer, QoS Baseline Proposal, IEEE 802.11 Standards Committee,Document IEEE 802.11-00/360 (Nov. 7, 2000); also QoS Baseline ProposalRevision 1, Document IEEE 802.11-00/360R1 (Nov. 7, 2000) and QoSBaseline Proposal Revision 2, Document IEEE 802.11-00/360R2 (Nov. 9,2000), the contents of which are hereby incorporated by reference. Someof the abbreviations and acronyms used in this application are:

-   -   ACK Acknowledgement;    -   AP Access Point;    -   BSS Basic Service Set;    -   BSSID Basic Service Set Identification;    -   CCA Clear Channel Assessment;    -   CFP Contention-Free Period;    -   CF-Pollable Contention-free Pollable CP Contention Period;    -   CW Contention Window;    -   DCF Distributed Coordination Function;    -   DIFS Distributed (coordination function) Interframe Space;    -   CW Contention Window;    -   DTIM Delivery Traffic Indication Message;    -   EAP Enhanced Access Point;    -   IFS Interframe Space;    -   MAC Medium Access Control NAV Network Allocation Vector;    -   PCF Point Coordination Function;    -   P-CFB Point-controlled Contention-free Bursts;    -   PIFS Point (coordination function) Interframe Space;    -   QoS Quality of Service;    -   SBM Subnet Bandwidth Manager;    -   SIFS Short Interframe Space;    -   STA Station;    -   V-DCF Virtual Distributed Coordination Function;    -   WSTA Wireless (enhanced) Station.

In the description that follows, an “outbound” transmission is atransmission from an AP and an “inbound” tansmission is directed to anAP.

The IEEE 802.11E QoS working group has adopted a baseline proposal(hereinafter “baseline proposal”) for channel access methods thatprovide QoS on 802.1 1 wireless LANs. The baseline proposal definesthree QoS levels - a “prioritised” DCF-based solution and “prioritised”and “parameterised” PCF-based solutions. It is generally agreed that asingle unified approach is better from a user perspective; however, nosingle approach, as it is currently defined, is applicable to allenvironments. A PCF approach is more deterministic and efficient insingle-BSS environments; however, it is difficult to implement a CFPscheduling algorithm in environments with BSS overlap. The presentinvention is directed to an integrated DCF/PCF channel access model thatuses V-DCF for lightly and moderately loaded channels, and dynamicallyuses unscheduled “Point-controlled Contention-free Bursts”, to arbitratechannel contention, as the network load increases.

The baseline “prioritised” solutions are intended for stations thatsimply send and receive prioritised frames. The baseline “parameterised”solution is intended for WSTAs that use a signaling protocol toestablish bandwidth requirements and delay constraints. This invention,the Point-controlled Contention Arbitration model, or PCCA model,requires all WSTAs to implement sufficient channel access and interfacefunctions to support optional parameterised services. QoS features canbe added to an AP implementation on an incremental basis.

The baseline proposal discloses a virtual DCF protocol (“V-DCF). Bydesign, the V- DCF, or level 1 in the baseline proposal, cannot provideintegrated services such as “Controlled Load” and “GuaranteedBandwidth”. Two fundamental requirements are lacking for controlled loadservice. 1) The total traffic at a given QoS priority must be limited(i.e. by admission control), and 2) higher priority traffic cannot beaffected by lower priority traffic. The V-DCF level cannot supportadmission control because it lacks even a simple signaling protocol. TheV-DCF access method, with contention offset and CWmin values percategory, only statistically increases the probability of channel accessfor higher-priority packets. A tiered channel access method can be usedto isolate a high-priority traffic category but only if the idle sensetime, required for any lower-priority traffic category, is greater thanthe sum of the idle sense time plus the maximum CW value for thehigh-priority category. However, the tiered method doesn't scale wellfor large high-priority populations. “Guaranteed Bandwidth” service hasthe same requirements as controlled load and also requires adeterministic channel access method.

Presently, 802.11 networks use two protocols, the DCF and PCF. The DCFworks great under low load situations. The PCF works optimal under highload conditions. The DCF works better in networks were BSSs overlap, thePCF is ideally suited for networks were BSSs are carefully planned notto overlap. The DCF has a relatively low implementation complexity, thePCF is reputed to be more complex to implement. The DCF does not allowexplicit access control, the PCF does. The DCF efficiency dropsconsiderably in densely populated BSSs, the PCF has no scaling problem.

Due to the inability of the PCF to work well under overlapping BSSconditions and the high implementation complexity, the PCF has not yetbeen widely adopted in current 802.11 implementations. The demand forbetter medium efficiency and a versatile QoS platform, however,increased interest in this optional access mechanism of the 802.11 MAC.

The hybrid nature of the 802.11 MAC has caused proposals to focus eitheron the DCF or the PCF. However, by only looking at the PCF and notconsidering the DCF overlooks the fact that the 802.11 MAC always spendssome time under the DCF access mechanism rules and that the DCF is alsoan integral part of a PCF based system. The system always has to spendat least a small part of its time under the DCF. The PCF has thefundamental characteristic that a station can't access the medium unlessexplicitly polled. However, to be polled, the station must first makeitself known to the Point Coordinator, which requires medium access.Therefore, a PCF based solution must support both contention-free andcontention periods. A contention period is required for bursty traffic,adjacent BSSes, probe requests, association and re-association requests,etc.

FIG. 1 shows an example of a sample rate for a real-time application ina WSTA and the associated polling sequences for that WSTA. The lowerportion 12 of FIG. 1 shows the WSTA sample rate. The upper graph 14shows the polling sequences. The polling sequence starts with a DTIMbeacon 16. The contention-free period 18 starts immediately after theDTIM beacon 16. During the contention-free period 18, the pointcontroller initiates polling sequences 20. After the polling sequences20 is shown an idle time period 22. The idle time period 22 is thenfollowed by an additional polling sequences 20 and idle time periods 22.Following the contention-free period 18 is the contention period 24.After the contention period 24, another DTIM beacon 16 starts a newsequence of a contention-free period 18 and a contention period 24.

In FIG. 1, the DTIM beacon rate is slower than the sampling rate. Idletime 22 is introduced if the CFP is extended so that the same WSTA canbe polled more than once per CFP. Latency is introduced if the channelis overloaded in the contention period.

Delay sensitive applications, such as VoIP, require short DTIM intervals(i.e. 30 milliseconds) to minimize CF polling latency. A fast DTIMbeacon rate wastes bandwidth because of the beaconing overhead andbecause contention-based transmissions cannot span the TBTT (per thebaseline proposal). A fast DTIM beacon rate also requires power-saveWSTAs to wake up more often, for example to receive multicast frames andbuffered unicast frames.

In installations with multiple QoS applications with different servicerates, the DTIM beacon rate cannot match the sampling rate for eachapplication. Actually, it is difficult to match the sampling rate forany application. It is not efficient to arbitrarily poll WSTAs in everyCFP. Therefore, some sort of signaling protocol is necessary to suppressunnecessary polls. In addition, a need exists for a protocol that candivorce the service rate, for active parameterised stations from thebeacon rate. Periodic polling is not optimal for intermittent traffic.VoIP traffic can be intermittent due to silence suppression.

Depending on the ‘load of the medium’, the system may spend more or lesstime in the CFP. In a heavily loaded system, the system may spend thelarger part in the CFP while a mildly loaded system may spend the largerpart in the CP. The balance between the two access mechanisms is afunction of the medium load. As a consequence, both access mechanismsmust provide the same QoS capabilities. The transition between oneaccess mechanism and the other must be a smooth one. This is especiallya challenge in average loaded systems where the DCF efficiency isstarting to breakdown while the PCF efficiency is not yet optimal. Forthe upper layer protocol (or application) the performance profile of theservice should be linear over all medium conditions and this issomething that should be considered when proposing a PCF based system.Therefore, when proposing PCF enhancements, one also to consider theinteraction between the PCF and the DCF and the dynamics of the systemas a whole under various medium load conditions.

PCF combines the ability of full medium control with optimal mediumefficiency, without suffering from scalability problems. However, thereare two issues that limit the use of the current PCF for QoS systems.Section 9.3.4 and specifically clause 9.3.4.1 of the IEEE 802.11standard imposes strict rules upon the order in which stations areaddressed or polled. This is undesirable in a QoS system. Secondly,there is no mechanism, other than the More- Data bit, that allows astation to communicate its queue states to the PC.

The entity in the PC that actually calculates the order in whichstations are addressed is in literature often referred to as the‘scheduler’. The rules for the handling of the polling list limit thefreedom of the scheduler and may conflict with QoS requirements. Theoriginal intent to poll stations in order of ascending AID value is notclear from the standard and in fact the whole concept of a polling listmay become obsolete due to the introduction of a mechanism forcommunicating To-DS queue state(s). Therefore, the rules as defined insection 9.3.4 are neglected for this method.

In order to make accurate scheduling decisions, the scheduler in the PCneeds to have knowledge about the queues in the associated stations. TheMore-Data bit is a Boolean that could be used for this but only allowscommunication of a truth-value on the queue state; for a good schedulerimplementation this is not enough. Preferably, the scheduler needs toknow the length and priority of the next frame in the queue of eachstation.

Scheduling problems arise with CFPs in networks with overlapping BSSesin the same ESS or multiple ESSes. A CFP is not completelycontention-free unless all stations in any neighboring BSS, that are inrange of any active stations in the BSS, set their NAV forCFPMaxDuration for the CFP. Therefore, the total “reservation area” fora CFP can be very large compared to the coverage area of the pointcontroller for the BSS.

The baseline proposal defines a “proxy beacon” mechanism where WSTAs ina BSS repeat AP beacons to extend the area for propagating beaconinformation to hidden nodes. The baseline proposal does not define whichWSTAs should send proxy beacons and it does not define the schedulingmechanism for proxy beacons. Also, it is not clear whether a hidden nodein a neighboring BSS, that receives proxy CFP beacons, should set itsNAV for CFPMaxDuration for the TBTT of the associated hidden CFP. Ifhidden nodes do not set their NAV for proxy beacons, then CFPs are notcontention-free.

If hidden nodes set their NAV for CFPMaxDuration for a hidden CFP (dueto proxy beacons or some other mechanism) then two difficult problemsmust be considered. First, as noted above, spatial reuse is severelyinhibited as compared to DCF. The baseline proposal attempts to solvethe “spatial reuse” problem by classifying WSTAs as belonging to overlapand non-overlap sets per BSS. However, that approach does not work forall applications because it assumes that a WSTA is relatively stablecompared to its transmission rate and it uses the flow error rate as anoverlap indicator. Second, if a hidden CFP ends early, then bandwidth iswasted because hidden nodes may not be able to determine that the hiddenCFP has ended. It has been suggested that WSTAs that transmit proxybeacons could also transmit “proxy CF-End” messages or CF-End messagescould be transmitted on the distribution system. The first solution is“chatty” and the second solution is not generally applicable because thedistribution system may introduce latency (i.e. if it includes wirelesslinks or IP tunnel links).

It should also be noted that in the PCF/CFP model, where the NAV is setfor long CFPs, use of sophisticated techniques that increase spatialreuse by varying the transmit power and/or antenna direction per unicasttransmission sequence is inhibited.

One suggested PCF enhancement that can ease the overlapping BSS problemand alleviate the scheduling problem is the Contention Free Burst. Apaper entitled “Suggested 802.11 PCF Enhancements and Contention FreeBursts”, IEEE 802.11-00/113 (May 10, 2000), written by Maarten Hoebenand Menzo Wentink, hereby incorporated by reference, describesbi-directional contention-free bursts that include point controllerpolling.

The Contention Free Bursts concept breaks up a Contention Free Periodinto smaller Contention Free Bursts. This is useful for two reasons:First, it allows the PC to relinquish medium control to other BSSs inthe same area. Second, in the case of average loaded systems, the PC cantemporarily give-up medium control (to possibly another BSS) and defercontrol until new frames are available for transmission.

Normally, a CFP starts with the transmission of a Beacon. A SIFS afterthe Beacon, the first CFB is started. Within the CFB, the PCF transferprocedures apply as defined in section 9.3.3 of the IEEE 802.11standard. CFBs have a maximum duration of CFBMaxDuration. The durationremaining in the CFB is encoded in the Duration/ID field of everyFrom-DS frame sent by the PC. The CFB may be foreshortened but neverlasts longer than CFBMaxDuration. The end of a CFB is signaled through aduration of 0.

Between two CFBs the PC performs a random backoff, selected from a rangeof 0 to CW-1 slots. The random backoff mechanism allows PCs to contendfor the medium to start a new CFB. In the current definition of the CFP,all stations (including other access points) set their NAV based on theDuration Remaining field in the CF-Parameter set and reset the NAV uponreceiving a CF-End. This prevents access points and stations fromaccessing the medium during observed medium idleness during the CFP(possibly caused by the transmission of a frame by a hidden node). PCsmay use the backoff mechanism to contend for the medium and start a CFPor continue their own CFP with a new CFB. In a sense the CFB conceptworks like a superimposed DCF over the PCF. PCs coordinate their burstsby using the backoff- mechanism, deferring and restarting the backoffswhenever a PC starts a CFB or ends the CFB. The CFBs are protectedthrough the NAV-alike duration field in the redefined in Duration/IDfield, CPs use the information as received in the ToDS frames from otherCPs to update their CF-Nav and defer backoff and start of a new CFB.

Note that only PCs contend for medium control; stations (and legacyaccess points) do not attempt to access the medium during the periods ofmedium silence caused by the backoff periods because they adhere to theContention Free Periods of (at least one of) the BSSs. A CFB isfurthermore protected from interference of legacy implementations due tothe SIFS/PIFS interframe spaces, and a Duration/ID field that isinterpreted as a very long NAV.

Another concern is that PCF and DCF applications do not always coexistwell. The PCF model only supports “polled” inbound transmissions duringa CFP. As a result long PCF-based CFPs can starve DCF-based stations.The problem is exacerbated when CFPs in overlapping BSSes must bescheduled to avoid CFP contention. PCF polling is appropriate forisochronous applications, but DCF is more appropriate for asynchronousdata. It should not be assumed that PCF polling is used for allhigh-priority inbound transmissions; however, the current baseline modelinherently prioritizes PCF over DCF. As an example, consider inboundasynchronous high-priority network control transmissions. Suchtransmissions can be delayed extensively by lower priority PCFtransmissions.

The current 802.11 standard specifies that an AP must buffer alloutbound multicast frames and deliver them immediately following a DTIMbeacon if “strict ordering” is not enabled, then. Therefore, short DTIMintervals are necessary to support multicast applications that cannottolerate delays. In addition, outbound multicast transmissions are moresusceptible to problems associated with inter-BSS contention and hiddennodes because multicast frames are not retransmitted (i.e. after acollision with a hidden node) and the DCF channel reservation mechanismscannot be used for multicast frames.

The baseline proposal removes the restriction that bufferedmulticast/broadcast frames must be sent immediately following a DTIMbeacon. The baseline proposal requires that QoS WSTAs must respond to+CF-Polls. Therefore, it is strongly recommended that QoS stationsshould also associate as CF-Pollable (i.e. not requesting to be polled).If QoS power-save WSTAs do not use the PS-Poll mechanism for thedelivery of outbound buffered messages, an AP can more easily scheduleoutbound transmissions for PS WSTAs. Note that CF-Pollable stations donot send PS-Poll frames to solicit outbound transmissions. Instead, aCF-Pollable station must stay awake, after it receives a DTIM beaconwith its AID bit set on, until either it receives a unicast frame withthe more bit set off, or a TIM with its AID bit set off. Therefore, itis generally assumed, but not required, that QoS WSTAs with active flowswill operate in active mode because a point controller cannotsuccessfully poll a WSTA that is in power-save mode.

The baseline proposal defines “awake-time epochs” that can, optionally,be used to set an awake-time window for periodic polling and/or outbounddata transmissions. However, awake-time epochs introduce complexity forP-CFB polling and PCF polling. If power management must be supported, itwould be simpler to schedule P-CFB polls for power-save parameterisedWSTAs, if such WSTAs used automatic power-save intervals. Such a power-save WSTA can remain in power-save mode for, at most, the duration ofits “automatic power-save interval”, following an inbound transmission,where the duration is selected to match the WSTAs inbound transmissionrate. The point controller can simply adjust the duration of the polltimer, for a WSTA, so that it is greater than the sleep-time windowduration. The point controller can then poll a WSTA and/or deliveroutbound buffered data for a WSTA when the poll timer expires.

Therefore, for the reasons set forth above, there is a need for acontention-based channel access method for supporting parameterised QoSapplications. Furthermore, the channel access method must coexist wellwith PCF and DCF applications.

BRIEF SUMMARY OF THE INVENTION

In view of the aforementioned needs, the invention contemplates acontention-based channel access method where the channel accesspopulation is divided into two distinct groups; the first group is a setof channel access arbitrators, and the second group is the set of allother stations. Channel access is highly prioritized for the channelarbitrators so that an arbitrator can quickly gain control of a (i.e.heavily loaded) channel during a contention period and mitigatecontention by directing (i.e. polling) which stations can access thechannel.

Another embodiment of this invention contemplates a multicastpower-management enhancement that can be used to reduce the DTIM beaconrate. In 802.11 networks, access points buffer multicast frames andtransmit those frames immediately following DTIM beacons, so that“power-save” stations can “sleep” between DTIM beacons without missingmulticast transmissions. Some time-bounded applications require a rapidDTIM beacon rate to avoid latency. As a result stations must wake upmore often and bandwidth is wasted because stations are prohibited fromtransmitting before a DTIM beacon.

Presently, outbound multicast transmissions are made only immediatelyafter a Delivery Traffic Indication Message beacon. The access pointmust buffer outbound multicast frames between DTIM beacons. Because thearbitrator is monitoring station service rates, the arbitrator knowswhen those stations being monitored are active on the channel andavailable to receive data from the arbitrator. Thus, the arbitrator maysend outbound messages independent of any beacons. By associating theactive stations to multicast groups, it may determined when all of thestations in the multicast group are available to receive frames.Therefore, an arbitrator can wait until all of the stations of amulticast group are active and broadcast a multicast frame independentof the DTIM beacon. Because the arbitrator has the capability to gaincontrol of a channel during the contention period, multicast frames fortime bounded applications may be transmitted during the contentionperiod, thus obviating the need for more frequent DTIM beacons. In thepreferred embodiment, the arbitrator would request that all stationsmust operate in active mode to join an “active multicast group.”

While the multicast transmission method can be utilized to increasepower savings, it is also contemplated that this method can be usedanytime a group of stations running a time bounded application needs toreceive a multicast frame. Divorcing the broadcast of multicast framesfrom the DTIM beacon, especially for parameterised QoS wireless stationsrequiring frequent servicing, can result in a reduction of the DTIMbeacon rate. Reducing the DTIM beacon rate has the additional benefit ofeasing channel congestion.

As with the power savings method, the access point buffers a multicastframe for a multicast group, associates the individual stations withtheir appropriate multicast groups, waits until every station of amulticast group is in an active mode, and broadcasting the frame.

Among those benefits and improvements that have been disclosed, otherobjects and advantages of this invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings. The drawings constitute a part of this specification andinclude exemplary embodiments of the present invention and illustratevarious objects and features thereof.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The drawing:

FIG. 1 shows an example of a sample rate for a real-time application ina WSTA and the associated polling sequence for that WSTA.

DETAILED DESCRIPTION OF INVENTION

The present invention contemplates a method for managing prioritizedchannel access. Prioritized channel access is required for parameterisedand prioritized stations. “Parameterised stations” are QoS stations withflows that require guaranteed bandwidth and bounded delays. It isassumed that such stations will use a signaling protocol, for exampleRSVP with SBM, to request a constant service rate. Prioritized stationsare QoS stations that transmit frames with a priority higher than “besteffort”, without using a signaling protocol to set delay and bandwidthparameters.

The method contemplates utilizing an AP channel access arbitrator thatmonitors the service rate for “parameterised QoS stations” and initiatesunscheduled Point-controlled Contention-free Burst (“P-CFB) polling, asrequired, during the contention period, to sustain a constant servicerate for such stations.

WSTAs may optionally use a signaling protocol to establish service rateparameters. The access arbitrator can use channel load feedback functionto estimate channel load and contention. It should be noted that thebaseline proposal requires an equivalent function in the AP to set CWminvalues per priority.

A P-CFB is essentially a contention-free burst that is extended toinclude PCF-like polling facilities. For outbound transmissions, thechannel access arbitrator gains control of the channel and beginstransmitting contention free bursts for the outbound transmissions. Forinbound transmissions, the channel access arbitrator gains control ofthe channel, polls the wireless station, and the wireless station'sresponse is a contention free burst. Therefore, P- CFB can consist ofone or more outbound transmissions, one or more polled inboundtransmission, or any combination of inbound and outbound transmissions,separated by a SIFS time. A P-CFB is not associated with a DTIM beacontransmission and stations do not preset their NAV for the maximumduration of a P-CFB.

In the PCCA model, WSTAs must support P-CFB polling and a functionalinterface that enables a signaling protocol to communicate service raterequirements to a bandwidth manager in the AP. It is intended thatparameterised services can be implemented by transparently layering asignaling protocol on top of the 802.11E protocol stack in a WSTA. Theuse of a signaling protocol is optional. Such a requirement isconsistent with the baseline proposal, which requires all WSTAs tosupport CF polling. It is necessary because PCCA polling must be drivenby the WSTA application transmission (i.e. sampling) rate to avoidarbitrarily polling WSTAs. The PCF polling rate is driven by the DTIMbeacon rate. It is relatively simple to support polling in WSTAs.

Minimum AP requirements are as defined in the baseline proposal. An EAPneed only support the level 1 V-DCF QoS protocol. An AP can optionallyimplement P-CFBs, CFPs, an overlap mitigation protocol, support for aQoS signaling protocol, and level 3 polling and TXOP enhancements. An APmust implement a signaling protocol and P-CFB polling to support aconstant service rate for each parameterised station.

CSMA channel efficiency can be very high, even under heavy load, if thecontention population is small. The PCCA model attempts to divide theentire station population into a small EAP population and a non-EAPstation population, for channel access purposes, so that an EAP candeterministically gain access to the channel, in the contention period,to transmit outbound frames or initiate a P-CFB. The EAP maintainscontrol of the channel during a P- CFB with the DCF CCA and DCF channelreservation mechanisms.

In one embodiment of this invention, the tiered channel access method,combined with some form of priority queuing, is all that is necessary toassure timely delivery of outbound high-priority unicast or multicastframes. CFPs can optionally be used to reduce contention, from hiddennodes, for outbound multicast transmissions associated with DTIMbeacons.

For inbound unicast transmissions, a channel access arbitrator in theEAP monitors the service rate for stations and initiates polling, asrequired, to maintain a constant service rate for parameterisedstations. Note that all inbound transmissions, in a BSS, are unicast. Ina simple implementation, the access arbitrator can maintain a “polltimer” for each parameterised station. A station is polled if the polltimer expires and the poll timer is reset each time the EAP receives aninbound frame from the station. The duration of the poll timer can beset long enough so that polling is never used on lightly, or moderatelyloaded channels and short enough so that the minimum delay for therespective flow is not exceeded. Note that a station can be polled ineither the optional contention-free period or the contention period.

An interactive voice session typically comprises of 2 fixed-rateintermittent flows. A flow periodically goes idle due to “silencesuppression”. For such applications, the channel access arbitrator canuse a channel load feedback function to monitor the channel load. Thearbitrator initiates P-CFB polling for such stations if 1) the polltimer has expired, and 2) the channel load is greater than the channelload threshold associated with the flow. On moderately loaded channels,the point controller will not waste bandwidth polling for inactiveflows.

Streaming video applications typically generate a constant stream ofvariable-sized compressed frames. Note that a single arbitrationalgorithm can support both VoIP and streaming video, simply by settingthe channel load threshold, for streaming video flows, to a low value(i.e. 0), to trigger P-CFB polling whenever the poll timer expires.

The use of the optional multi-poll mechanism, is not prohibited during aP-CFB. However, simple, explicit polling works better with variable rateflows, for example streaming video, and explicit polling can helpprevent interference from hidden nodes.

The point controller does not necessarily know the duration of aninbound transmission associated with a P-CFB poll. Therefore, the DCFchannel reservation (i.e. in the Duration/ID field) in a P-CFB poll mustbe for a time slightly longer than the worst-case maximum fragmenttransmission time. A WSTA should adjust point controller channelreservations, as is appropriate. For example, a WSTA should cancel apoint controller reservation, if it receives a unicast frame from thepoint controller, where the RA address matches the WSTA address. A WSTAshould shorten its reservation if receives a frame from the pointcontroller, where the reservation is shorter, and the RA address doesnot match.

Simple P-CFB polling sequences, that consist of 1) an AP poll, 2) aW-STA data frame, and 3) and an AP ACK, work well in environments withhidden nodes. The reservation in the initial AP poll frame reserves thechannel, in the coverage area of the AP, for the duration of the,possibly hidden, data transmission from the WSTA. The final AP ACKtransmission cancels the reservation (i.e. which may exceed the durationof the, possibly null, data transmission).

The hidden node problem is exacerbated by WSTAs that change frequenciesor wake up, sense the channel idle for a DIFS time, and transmit. SuchWSTAs may miss an initial poll or CTS frame that preceded a transmissionfrom a hidden WSTA. The hidden node problem can be partially addressedby limiting the maximum duration of inbound transmissions so thatunicast transmission sequences consist of alternating AP transmissionsand bounded WSTA transmissions. Interleaved AP polls, for example, canbe used to sustain the channel reservation at the AP during P-CFBpolling in the contention period. WSTAs are initially required to sensethe channel for a time slightly greater than the maximum transmissionduration of an inbound fragment, where a fragment can be a partial frameor a whole frame, after first waking up or changing frequencies. U.S.Pat. No. 5,673,031, hereby incorporated by reference, describes such aprotocol. Note that the channel reservation at the AP cannot besustained for unbounded back-to-back TXOPs, with either delayed ACKs orno ACKs.

A WSTA “queue feedback mechanism” enables the point controller todetermine the priority queue state in QoS stations, so that the pointcontroller could use priority scheduling for inbound transmissions. Sucha feedback mechanism would be useful for ordering polls and avoidingunnecessary polls. For example, the channel access arbitrator couldreset its poll timer for a station if an ACK from the station indicatedthat it did not have data queued.

It might also be useful to include a “priority token” on outboundunicast data frames. For example, a QoS station could respond to anoutbound unicast transmission, where the RA address matched the stationaddress, with an inbound transmission, with a piggybacked ACK, if it hadan equal or higher priority data frame queued. Such a mechanism would beuseful for maintaining a constant service rate, without explicitpolling, for applications with constant bi- directional flows (i.e.interactive voice without silence suppression).

The following channel access rules are used to implement the tieredchannel access method for the contention period for the preventinvention. An EAP can use the tiered access method to gain control ofthe channel for the transmission of any outbound frame (i.e. beacon,data, and management frames) or to initiate a P-CFB. Note that acontention-free burst, as defined in the baseline proposal, can beregarded as a special case of a P-CFB. First, an EAP can access thechannel during the contention period (CP) after the channel is idle fora SIFS time following an inbound or WSTA-to-WSTA transmission sequenceinitiated by a WSTA within the BSS controlled by the EAP. Second, CWminvalues can be set differently for EAPs and WSTAs to prioritise EAPchannel access, as defined in the baseline proposal. Third, an EAP mustonly sense the channel idle for a PIFS time before initiating thepost-backoff following a successful or unsuccessful single-frame orburst transmission. QoS WSTAs and legacy WSTAs must sense the channelidle for a DIFS time. Fourth, an EAP must only sense the channel idlefor a PIFS time before restarting its backoff countdown, following abusy channel sense. Fifth, the configuration variable that controls themaximum duration of a P- CFB is the same as the variable that controlsthe maximum duration of an AP contention-free burst, as defined in thebaseline proposal. Sixth, the DCF access mechanisms (channel reservationand CCA) are used to control the channel during a P-CFB. Bit 15 is setto 0 in the Duration/ID field, in a frame transmitted during a P-CFB, toindicate that the field contains a valid channel reservation value.

It has been noted that 802.11 CSMA “slot” times can be ambiguous. In thecontention period, stations that are waiting to access a busy channelmust sense the channel idle for a fixed time before restarting thebackoff countdown. In the integrated mode, the idle sense time is a PIFStime for EAPs and a DIFS time for other WSTAs. The end of a transmissionprovides a “slot synchronization point” for stations waiting to accessthe channel. The efficiency of a CSMA algorithm can be greatly increasedif stations transmit on slot boundaries (i.e. following the end of atransmission). However, the present invention is not limited to thismethod.

Another aspect of the present invention is power management. The PCCAmodel adheres to the channel access rules defined in the 802.11 standardand the baseline proposal. The baseline proposal removes the restrictionthat buffered multicast/broadcast frames must be sent immediatelyfollowing a DTIM beacon. If “strict ordering” is not enabled, then thecurrent 802.11 standard specifies that an AP must buffer all outboundmulticast frames and deliver them immediately following a DTIM beacon.Therefore, short DTIM intervals are necessary to support multicastapplications that cannot tolerate delays.

The baseline proposal requires that QoS WSTAs must be CF-Pollable.Therefore, QoS power-save WSTAs do not use the PS-Poll mechanism for thedelivery of outbound buffered messages. Instead, a QoS PS WSTA mustadhere to the existing standard for CF- Pollable stations. That is, itmust stay awake, after it receives a DTIM beacon with its AID bit seton, until either it receives a unicast frame with the more bit set off,or a TIM with its A/D bit set off.

It is generally assumed, but not required, that QoS WSTAs with activeflows will operate in active mode, because a point controller cannotsuccessfully poll a WSTA that is in power-save mode. The baselineproposal defines “awake-time epochs” that can, optionally, be used toset an awake-time window for periodic polling and/or outbound datatransmissions.

However, awake-time epochs introduce complexity for P-CFB polling andPCF polling. If power management must be supported, it would be simplerto schedule P-CFB polls for power-save parameterised WSTAs, if suchWSTAs used automatic power-save intervals, where power-save intervalsare defined as follows: Such a power-save WSTA can remain in power-savemode for, at most, the duration of its “automatic power-save interval”,following an inbound transmission, where the duration is selected tomatch the WSTAs inbound transmission rate. The WSTA must operate inactive mode, after a power-save interval expires, until the end of thenext polling sequence or inbound transmission. The point controller cansimply adjust the duration of the poll timer, for a WSTA, so that it isgreater than the sleep-time window duration. The point controller canthen poll a WSTA and/or deliver outbound buffered data for a WSTA whenthe poll timer expires.

If “strict ordering” is not enabled, then the current 802.11 standardspecifies that an AP must buffer all outbound multicast frames anddeliver them immediately following a DTIM beacon. Therefore, short DTIMintervals are necessary to support multicast applications that cannottolerate delays.

A layer 2 multicast group is typically associated with a single higherlayer application such as example streaming video. The present inventionsupports “power-save” and “active” multicast addresses, where amulticast address is classified as “power-save” if any station in themulticast group is in power-save mode. Then outbound frames destined toan “active” multicast RA address can be delivered immediately.Therefore, the DTIM interval can, potentially, be much longer. Note thata multicast registration protocol (i.e. GMRP) is required to associatemulticast addresses with stations.

It should be noted that outbound multicast transmissions are moresusceptible to problems associated with inter-BSS contention and hiddennodes because multicast frames are not retransmitted, for example aftera collision with a hidden node, and the DCF channel reservationmechanisms cannot be used for multicast frames. In the method of thepresent invention, CFPs are primarily used to increase the reliabilityfor multicast transmissions. A CFP scheduling algorithm can be used toprevent CFPs in adjacent BSSes from colliding.

In a single-BSS environment, the tiered access method enables an EAP toquickly access the channel in the contention period, with a worst-caselatency equal to the maximum duration of a 2304-byte transmissionsequence. Note that the tiered channel access method allows the EAP touse CWmin values of 0 in the absence of channel contention from otherAPs. The EAP uses the DCF CCA and DCF channel reservation mechanisms tomaintain control of the channel during a P-CFB. SIFS frame spacing isused for both P-CFBs and WSTA CFBs. The maximum duration of a P-CFB canbe increased to improve channel efficiency. Therefore, the method of thepresent invention is comparable to a PCF-based solution with respect todeterminism and channel efficiency.

Networks with multiple and overlapping BSSes may have problems withhidden CFP's or hidden nodes. A “hidden CFP” is a CFP in a first BSSwhere the point controller is not within the range of a station in asecond neighboring BSS. Likewise, a “hidden node” is a station in afirst BSS that is not within the range of the point controller for a CFPin a second neighboring BSS. In environments with multiple, overlappingBSSes, the present invention reduces inter- BSS contention and increasesspatial reuse because it relies on short randomly-spaced DCF- basedP-CFBs, with short localized reservations, rather than long scheduledPCF-based CFPs. The CFP rate can be decreased, and the inter-DTIM periodcan be increased because the CFP rate is independent of the applicationsampling rates.

In networks with overlapping BSSes in the same ESS or multiple ESSes, aCFP for a BSS is not completely contention-free unless all stations, inany neighboring BSS, that are in- range of any active stations in theBSS, set their NAV for CFPMaxDuration for the CFP. Therefore, the total“reservation area” for a CFP can be very large compared to the coveragearea of the point controller for the BSS. In contrast, the reservationarea for a P-CFB unicast transmission sequence is limited to thecoverage area of the respective stations and the reservation iscancelled when the transmission sequence ends.

1. A method of power management for a network, comprising: buffering aframe for a multicast group; associating a station with the multicastgroup, the station having an automatic power save interval and being ina power save mode during the power save interval and in an active modeotherwise; monitoring to determine when the station is in an activemode; waiting until every wireless station of the multicast group is inthe active mode; and, broadcasting the frame.
 2. The method of powermanagement for a network as in claim 1, wherein the station iscontention free pollable.
 3. The method of power management for awireless network as in claim 2, wherein the wireless station is inactive mode when it is available for a contention free poll.
 4. Themethod of power management for a network as in claim 1, wherein a GARPMulticasting Registration Protocol is used to associate the station withthe multicast group.
 5. The method of power management for a network asin claim 1, the monitoring further comprises initializing a timer forthe station and determining the station is in the active mode after thetimer expires.
 6. The method of power management for a network as inclaim 5, further comprising resetting the timer upon receiving aninbound frame from the station.
 7. The method of power management for anetwork as in claim 5, the monitoring further comprising associating aseparate timer for every station in the multicast group and determiningevery station in the multicast group is awake upon the expiration ofevery associated separate timer.
 8. The method of power management for anetwork as in claim 7, further comprising resetting an associated timerfor a station upon receiving an inbound frame from the station.
 9. Themethod of power management for a network as in claim 1, wherein thebroadcasting of the frame occurs during a contention period.
 10. Themethod of power management for a network as in claim 6, furthercomprising waiting for an inbound packet from a wireless stationimmediately after sending the broadcast packet.
 11. A method of powermanagement for a wireless station, comprising: starting a timer;entering a power save state; exiting the power save state after thetimer expires; waiting for a transmission; and sending a transmissionimmediately after waiting for a transmission without waiting apredetermined time interval and without sensing the media.
 12. Themethod of power management of claim 8, wherein the predetermined timeinterval is an interframe space that is one of the group consisting of ashort interframe space, a distributed interframe space and a pointinteframe space.
 13. An access point, comprising: the access pointconfigured to associate a first wireless station and a second wirelessstation into a multicast group, wherein the first wireless stationhaving a first power save interval corresponding to a time period thatthe first wireless station is in a power save state and the secondwireless station having a second power save interval corresponding to atime period that the second wireless station is in a power save state;the access point configured to set a first timer corresponding to thefirst power save interval and a second timer corresponding to the secondpower save interval; the access point configured to buffer a frame forthe multicast group; the access point configured to broadcast the frameafter the first timer and the second timer have expired.
 14. The accesspoint of claim 13, wherein a GARP Multicasting Registration Protocol isused to associate the first wireless station and the second wirelessstation with the multicast group.
 15. The access point of claim 13,further comprising the access point configured for at least one of thegroup consisting of resetting the first timer upon receipt of an inboundframe from the first wireless station and resetting the second timerupon receipt of an inbound packet from the second wireless station. 16.The access point of claim 13, further comprising, reserving a wirelesschannel after the first timer expires; sending a unicast frame with apoll to the first station after the first timer expires; and waiting fora response for the poll.
 17. A wireless station, comprising: thewireless station configured to start a timer corresponding to a powersave state; the wireless station configured to entering a power savestate; the wireless station configured to exiting the power save stateafter the timer expires; the wireless station configured to wait for atransmission; and the wireless station configured to sending an inboundframe immediately after waiting for a transmission without waiting apredetermined time interval and without sensing the media.
 18. Thewireless station of claim 17, wherein the predetermined time interval isan interframe space that is one of the group consisting of a shortinterframe space, a distributed interframe space and a point inteframespace.
 19. An access point, comprising: means for associating a firstwireless station and a second wireless station into a multicast group,wherein the first wireless station having a first power save intervalcorresponding to a time period that the first wireless station is in apower save state and the second wireless station having a second powersave interval corresponding to a time period that the second wirelessstation is in a power save state; means for setting a first timercorresponding to the first power save interval; means for setting asecond timer corresponding to the second power save interval; means forbuffering a frame for the multicast group; and means for broadcastingthe frame after the means for setting a first timer and the means forsetting a second timer have expired.
 20. An access point according toclaim 19, further comprising: means for resetting the first timer uponreceipt of an inbound packet from the first wireless station; and meansfor resetting the second timer upon receipt of an inbound packet fromthe second wireless station.